In recent years, in order to improve the fuel efficiency of automobiles and the like and enhance the corrosion resistance and the like, a high-strength plated steel sheet obtained by subjecting a high-tension steel sheet to hot-dip galvanizing or hot-dip galvannealing is widely used. As a high-strength plated steel sheet, a steel sheet having an improved property such as strength, ductility, or formability by addition of Si or Mn is increasingly used.
Meanwhile, Si and Mn are easily oxidizable elements that are more likely to be oxidized than Fe is. For this reason, even in a reducing atmosphere for Fe, Si and Mn are often easily oxidized and concentrated on the steel sheet surface to form oxides. These oxides considerably decrease the plating wettability between molten zinc and the steel sheet surface at the time of plating. This causes occurrence of bare spot or white spot, thereby deteriorating an outer appearance of the plated steel sheet.
Therefore, various methods are proposed in order to produce a high-strength plated steel sheet suppressed in occurrence of bare spot accompanying the addition of Si or Mn. For example, Patent Literature 1 proposes a method for producing a hot-dip galvanized steel sheet containing Si and Mn and being excellent in plating wettability and pickup resistance. Specifically, this literature discloses a method in which, in continuous hot-dip galvanizing equipment provided with a heating furnace of all radiant tube type, annealing is carried out sequentially through the steps of preheating, heating, and soaking while a logarithmic ratio of a hydrogen partial pressure to a water vapor partial pressure within the furnace is controlled.